Aluminum alloy advanced welding process

Aluminum alloys are widely used in various welded structural products because of their light weight, high specific strength, good corrosion resistance, non-magnetic properties, good formability, and good low-temperature properties. Aluminum alloys are used instead of steel plate materials for structural weight. Can reduce more than 50%.

There are several difficulties in welding of aluminum alloys: 1 The welded joints of aluminum alloys are softened and their strength coefficient is low. This is also a major obstacle to the application of aluminum alloys; 2 The surface of aluminum alloys is susceptible to refractory oxide films (Al2O3 has a melting point of 2060 °C). , This requires the use of high power density welding process; 3 aluminum alloy welding easy to produce pores; 4 aluminum alloy welding is easy to produce hot cracks; 5 linear expansion coefficient is large, easy to produce welding distortion; 6 aluminum alloy thermal conductivity (about 4 times that of steel, the heat input is 2 to 4 times larger than the welded steel at the same welding speed.

Therefore, the welding of aluminum alloy requires the use of an efficient welding method with a large energy density, a small welding heat input, and a high welding speed.

2 Advanced welding process of aluminum alloy

In view of the difficulties in welding of aluminum alloys, several new technologies have been proposed in recent years. They have been applied in the transportation, aerospace, and aviation industries. Several new processes can solve the difficulties in welding of aluminum alloys, and the joint performance after welding is good. , And can weld the aluminum alloy that has not been welded or welded before.

2.1 Friction stir welding of aluminum alloy

Friction Stir Welding (FSW) is a new solid-state plastic joining process proposed by the Welding Institute (TWI) in 1991. Friction stir welding work principle is to use a special type of mixing head to insert the workpiece to be welded, through the high-speed rotation of the stirring head and the workpiece between the friction stir, friction heat to make the metal in the thermoplastic state, and the pressure in the stirring head The plastic flow from the front to the back of the lower part causes the weldment to be pressure welded together. Since there is no melting of metal in the friction stir welding process, it is a solid-state connection process, so there are no welding defects during welding, and it is possible to weld non-ferrous metal materials such as aluminum and high-strength aluminum alloy that are difficult to be welded by the welding method. , copper alloys, titanium alloys, dissimilar materials, composite materials, etc. At present, friction stir welding is used in the research and application of aluminum alloy welding. Aluminum alloys that have been successfully friction stir welded include 2000 series (Al-Cu), 5000 series (Al - Mg), 6000 series (Al - Mg - Si), 7000 series (Al - Zn), 8000 series (Al - Li) etc. Already abroad. Into the industrial production stage, this technology has been applied in Norway to weld a 20-m long structural part of a yacht. The Lockheed Martin Aerospace Corporation of the United States used this technology to weld cryogenic vessel rocket structural parts that store liquid oxygen in aluminum alloys.

The aluminum alloy friction stir welding seam is formed by plastic deformation and dynamic recrystallization. The grain size of the weld zone is refined. The dendritic grains without welding are dense and the microstructure is dense. The heat affected zone is narrower than the fusion welding, and no alloying elements are burned. Defects such as cracks and blowholes provide good overall performance. Compared with the traditional welding method, it has no spatter and smoke, no need to add welding wire and shielding gas, and the joint performance is good. Due to the solid-phase welding process, the heating temperature is low, and the microstructure of the welding heat affected zone has little change. For example, the metastable phase remains basically unchanged, which is very favorable for heat-treated aluminum alloys and precipitation-strengthened aluminum alloys. The residual stress and deformation after welding are very small, and the aluminum alloy sheet is basically not deformed after welding. Compared with ordinary friction welding, it can not be limited by the shaft parts, can weld straight welds, fillet welds. The traditional welding process for welding aluminum alloys requires removal of the oxide film on the surface and processing within 48 h. However, the friction stir welding process can remove oil contamination before welding, and the assembly requirements are not high. And friction stir welding saves energy and pollution compared to melting welding.

Friction stir welding aluminum alloy also has some shortcomings: 1 aluminum alloy friction stir welding speed is lower than the melting of welding; 2 weldment clamping requirements are high, the welding process requires a certain amount of pressure on the weldment, the back requires a backing plate; 3 The welding head at the rear end of the welding head forms a residual hole in the agitator head, which generally requires welding or mechanical removal. 4 The adaptability of the agitating head is poor. Different thicknesses of the aluminum alloy sheet require different structures of the agitating head, and the agitating head wears fast; 5 The process is not yet Mature, currently limited to structurally simple components such as flat structures, circular structures. Friction stir welding process parameters are simple, mainly including the rotating speed of the stirring head, the moving speed of the stirring head, the pressure of the welding part and the size of the stirring head.

2.2 Laser Welding of Aluminum Alloys

Aluminum and aluminum alloy laser welding technology (Laser Welding) is a new technology developed in the past decade, compared with the traditional welding process, it has strong features, high reliability, no need for vacuum conditions and high efficiency. Its advantages include large power density, low total heat input, deep penetration of the same heat input, small heat affected zone, small welding distortion, high speed, and easy industrial automation. It is especially advantageous for heat treatment aluminum alloys. It can increase the processing speed and greatly reduce the heat input, thus improving the production efficiency and improving the welding quality. When welding high-strength and high-thickness aluminum alloys, the traditional welding method is impossible to achieve single-pass penetration. However, when the laser deep-fusion welding forms a large depth of keyholes, a keyhole effect may occur, which can be achieved.

Laser welding aluminum alloy has the following advantages: 1 high energy density, low heat input, small thermal deformation, narrow melting zone and heat affected zone and large penetration depth; 2 high cooling rate to obtain fine weld microstructure, good joint performance; 3 Compared with the contact welding, laser welding does not use electrodes, so reducing the man-hours and costs; 4 does not require vacuum atmosphere when electron beam welding, and protective gas and pressure can be selected, the shape of the workpiece is not affected by electromagnetic, does not produce X Ray; 5 can be sealed inside the transparent object of metal material welding; 6 laser optical fiber can be used for long-distance transmission, so that the process adaptability, with the computer and robot, can achieve the welding process automation and precision control.

The currently used lasers are mainly CO2 and YAG lasers. The power of CO2 lasers is large, and it is suitable for thick plates that require high power. However, the absorptivity of the CO2 laser beam on the aluminum alloy surface is relatively small, resulting in a large amount of energy loss during the welding process. The YAG laser has a relatively low power, and the absorption rate of the YAG laser beam on the surface of the aluminum alloy is relatively larger than that of the CO2 laser, and can be conducted by the optical fiber, and the adaptability is strong and the process arrangement is simple.

When welding large-thickness aluminum alloys, traditional welding methods are impossible to achieve single-pass penetration. However, when deep penetration welding of lasers forms large-depth keyholes, the occurrence of keyhole effects can be achieved.

The difficulty in laser welding of aluminum and aluminum alloys lies in the fact that the absorption of radiant energy by aluminum and aluminum alloys is weak, and the initial surface absorption of CO2 laser beam (wavelength of 10.6 μm) is 1.7 %; for YAG laser beam (wavelength is 1. The absorption rate of 06 μm) is close to 5%. It is more complicated, causing electrode burnout and arc swing during high-frequency arc ignition. The stability after arcing is not strong, and at the same time, the electrode burns rapidly under high temperature of the arc. However, the combination of laser and plasma arc can significantly increase the penetration and welding speed.

There are many controllable parameters in the aluminum alloy laser-arc hybrid welding process, which mainly includes the following aspects. 1 Laser power, arc current and voltage, etc.: Hybrid welding reduces the laser power requirements, and the power factor has a great influence on the process. The greater the laser power, the greater the penetration depth, and the influence is far greater than the penetration depth when the laser welding alone. The effect of increasing the arc power supply power, increasing the width of the melting zone and increasing the heat-affected zone, if pulsed YAG laser is used, the pulse frequency and width can be adjusted to improve the process stability and reduce the formation of pores; 2 Welding speed parameters: with welding The increase in speed, welding heat input decreases, weld penetration decreases, and different welding speeds affect the role of the keyhole is different, thus affecting the stability of the welding; 3 laser and arc center distance: within a certain range, the laser The smaller the arc center distance DLA11 is, the greater the penetration is. At this time, increasing the arc current not only increases the melting width, but also increases the penetration. 4 Laser and arc coordination methods: Internationally, the research on composite welding generally adopts laser vertical incidence, and arc The laser beam is angled, different designs are arranged along the welding direction or before or after the arc. The process stability of composite welding and the formation of welding pores and cracks; 5 Effect of filling materials: Filling of welding wire, powder to supplement the burning of alloying elements, increase weld strength, improve process performance, and prevent hot cracking; 6 Protection of gases Composition and flow rate: The protective gas in hybrid welding is generally Ar, He or Ar/He mixed gas. Ar ionization energy is low, it is easy to form plasma, and it forms a coupling with laser beam photons, which is not conducive to protection, so pure He gas ratio is pure. Ar gas protection effect is good, but from the economic point of view, Ar gas is more economical, foreign use of Ar75% + He25% gas mixture protection for laser welding, the effect is good, and can improve process performance. There are other factors, such as the cleanliness of the aluminum alloy surface before welding, the removal of the oxide film, and post-weld heat treatment. When welding high-strength thick-plate aluminum alloys, multi-pass welding processes can be used to achieve full penetration welding. However, thick aluminum alloy welding is prone to problems such as porosity, hot cracks, and weld softening, and the process is complicated. Thick aluminum alloy welding deformation is serious, so it must adopt some anti-deformation process.

2.4 Electron Beam Welding of Aluminum Alloys

Electron beam welding refers to a welding method in which a converged high-speed electron beam is used to bombard heat energy generated at a joint of a workpiece in a vacuum environment to weld the welded metal. Electron beam as a welding heat source is characterized by high power density, strong penetration, accurate, rapid, controllable, and good protection. For the electron beam welding of aluminum alloys, the high energy density can greatly reduce the heat affected zone, improve the strength of welded joints, and avoid the occurrence of defects such as hot cracks. Because of its high energy density, it has a strong penetrating ability to weld hard aluminum alloy thick plates.

Compared with the traditional arc welding aluminum alloy, the electron beam welding energy density is 3-4 orders of magnitude higher than that of another high energy density welding process, laser welding. Therefore, the heat affected zone of the welded joint is very small, and the joint strength is much higher than the conventional welding method. The electron beam has good penetrability and can be applied to aluminum alloys with a large thickness. The mechanical properties of the joint after welding are good. The crack resistance of aluminum alloy weld metal increases with increasing weld energy density and heat input. Therefore, the anti-cracking performance of aluminum alloy electron beam welded joints is much higher than that of conventional welded joints, and is generally 1 to 1.5 times higher than that of argon arc welding. After the electron beam welding of the aluminum alloy, the residual stress is small and the deformation is small, and it is almost impossible to deform the thin plate after welding. Electron beam welding is required to be completed under vacuum conditions. Vacuum is a better protection method. Under this condition, pure weld metal can be obtained and air or shielding gas pollution can be avoided. When the electron beam welding aluminum alloy is vacuum remelted, the content of impurities in the weld is minimal, and the weld gas content is reduced by nearly half, so that the plasticity and toughness of the weld are greatly improved. The electron beam has good controllability and can be conveniently scanned, deflected, tracked, etc., and it is easy to automate the welding process. The electron beam scanning bath can eliminate defects and improve the quality of the joints.

Electron beam welding is a more effective method of obtaining an excellent weld seam by scanning the weld just after welding. The retrace distance determines the controllability of the grain refinement, and the solidification structure can be converted into a fine equiaxed crystal by a large columnar crystal. Compared with non-scanning welding of AlMg0.4Si1.2 alloys, the length of the crystal major axis is reduced to 1/5 of that without a scanning weld; the weld hardness is increased by 80%, close to the base metal level. The degree of grain refinement of the aluminum alloy weld metal has an important influence on the joint performance. The use of electron beam scanning welding with a flyback motion reduces the loss of alloying elements, refines the microstructure of the weld seam, turns it into fine equiaxed grains, and increases hardness. For crystals that have nucleated growth, if the electron beam scanning pitch is too small, remelting occurs during electron beam scanning, but the effect of causing the electron beam to sweep back to refine the crystal grains is weakened.

The electron beam welding of the aluminum alloy is very sensitive to the electron beam flow, especially when the aluminum alloy plate with a large thickness is welded, the electron beam current can not be penetrated through for a long time, and when it is large, it will fall down and appear pits. Another difficulty in electron beam welding of aluminum alloys is the welding of air holes. The main components of the oxide film on the aluminum alloy surface are Al2O3 and MgO, and it is easy to absorb a large amount of moisture, which is the main source of the pores in the aluminum alloy weld. The specific gravity of the aluminum oxide film on the surface of the aluminum alloy is close to that of the substrate, and the inclusions and pores can easily enter the weld seam. In particular, rust-proof aluminum alloy electron beam welding, the problem of stomatal is more serious. Traditional TIG welding aluminum alloy usually adopts large heat input and welding at lower welding speed, which promotes hydrogen to escape from the molten pool. While electron beam welding of aluminum alloy is fast, the heat input is small, and hydrogen is too low. Escape from the molten pool, easy to form pores. Normally, electron beam welding of aluminum alloys employs subsurface focusing and narrower welds and scanning remelting methods to prevent pinholes from being generated. In addition, electron beam welding is required under vacuum conditions, so it is difficult to weld large aluminum alloy structural parts. The electron beam is easily affected by the electromagnetic field of the surrounding environment. The equipment is relatively complicated and the cost is relatively expensive, so it has not yet reached large-scale industrial production.

The partial vacuum electron beam welding process developed in recent years has solved the problem of aluminum alloy electron beam welding of large components. Drauge2lates et al. successfully performed partial vacuum high-speed electron beam welding on AlMg5Mn and AlMg0.4Si1.2 alloys. The results show that welds without welding defects can be produced by welding at a high speed of 60 m/min, and local vacuum electron beams can be seen. Welding aluminum alloy has a very good prospect of development and is an advanced process for welding aluminum alloys.